The hitherto-known optical liquid sensor comprises a mirror and a beam splitter as illustrated in FIG. 19. Here, a light output 72 of a light-emitting element 71 passes through a beam splitter 73 (or a photocoupler) and an optical fiber 75 accommodated in a flexible duct 74 and reaches a detection means 76. The detection means 76 comprises said optical fiber 75 and a mirror 77 juxtaposed with an intervening gap .epsilon. which is to be filled up with a liquid substance to be tested (hereinafter referred to as the test liquid) A transmission loss of light attributable to the test liquid occurs in the course of travel of light 72 as it is reflected by the mirror 77 and returns to the optical fiber 75. A reflected light 78 returning to the optical fiber 75 is reflected by the beam splitter 73 and impinges on a light-receiving element 70, the output of which is amplified by an amplifier 80 for detection. This light output has been attenuated according to the transmission loss caused by the test liquid in the gap .epsilon. and the degree of contamination or the concentration of the liquid is determined from the degree of the above attenuation.
In the above optical liquid sensor employing a mirror and a beam splitter, reflection losses at the mirror 77 and beam splitter 73 are fairly large and the sensitivity of the system is as much decreased. Therefore, the use of a high-sensitivity amplifier 80 is essential. Moreover, an expensive beam splitter 73 is an absolute necessity. Thus, the initial cost of the system is high. Furthermore, in the system employing a mirror 77, the distance of travel of light is 2.times..epsilon.. Therefore, when the test liquid is one of large transmission loss, the gap .epsilon. must be so small that it is difficult to fill and remove the test liquid with respect to the gap .epsilon..
Having been accomplished to overcome the abovementioned disadvantages, the present invention has as its object to provide an optical liquid sensor which is highly sensitive and provides ease of filling and removal of the test liquid.
It is another object of the present invention to provide a method of manufacturing such an optical liquid sensor.
A still another object is to provide an automotive oil/battery checker employing the sensor.